1. Field of the Invention
The present invention relates to charged particle imaging using electron beam microcolumns and in particular a cleaning method and system for cleaning the silicon microlenses in the microcolumns.
2. Description of Related Art
In recent years, there has been a significant increase in interest in low voltage scanning electron microscopy (SEM) for applications in surface inspection of materials, metrology, testing and lithography.
Conventional scanning electron microscopes are large immobile devices. Although scanning electron microscopes have many applications, such as semiconductor related inspection and testing, conventional scanning electron microscopes are limited in their usefulness because of their size, immobility, and associated costs. For instance, because the sample being observed, as opposed to the electron microscope, must be moved during the inspection process, a conventional scanning electron microscope requires the use of a vacuum chamber that is much larger than the sample. Further, the sample must be positioned at an angle relative to a conventional scanning electron microscope to produce a beam incidence angle required for three-dimensional-like surface feature imaging, which makes handling large or delicate samples difficult. Moreover, throughput of a conventional electron microscope is limited because only one electron microscope can observe a sample at a time.
An effort to improve electron-beam systems has resulted in miniature electron-beam microcolumns ("microcolumns"). Microcolumns are based on microfabricated electron "optical" components and field emission sources operating under principles similar to scanning tunneling microscope (STM) aided alignment principles. Also called STM aligned field emission (SAFE). The alignment principles used by microcolumns are similar to STMs in that a precision X-Y-Z positioner is used to control a sharp tip, and to utilize the emission from the tip to measure the position of the tip. Microcolumns are discussed in general in the publication "Electron-Beam Microcolumns for Lithography and Related Applications," by T. H. P. Chang et al., Journal of Vacuum Science Technology, Bulletin 14(6), pp. 3774-81, November/December 1996, which is incorporated herein by reference.
The microcolumns are formed of high aspect ratio micromechanical structures, including microlenses and deflectors. The microlenses are multi-layers of silicon, including silicon membrane windows (apertures) which act as the microlens electrodes. The layers are spaced apart with and typically bonded to thick one hundred (100) to five hundred (500) microns insulating layers. The lenses have apertures with diameter that vary from about two (2) to two hundred (200) microns. For optimum performance, the apertures or bores are required to be formed with an interior bore smoothness in the nanometer regime and the alignment required is on the order of less than one micron. No tolerance for contamination of the lens aperture can be allowed.
It thus would be desirable to achieve cleaning of the silicon microlenses in situ during operation of the microcolumns.